1. Field of the Invention
The present invention relates to a method of manufacturing a ceramic laminate which is adapted to provide a laminated ceramic capacitor, a laminated LC composite part, a ceramic multilayer substrate or the like, and more specifically, it relates to a method of manufacturing a laminate by stacking ceramic green sheets.
2. Description of the Background Art
A component such as a laminated ceramic capacitor or a ceramic multilayer substrate, for example, has a laminated structure comprising ceramic sheets provided with an internal electrode which is interposed at least one interface, between at least two of the sheets. Such components have been generally obtained by applying ceramic slurry on one surface of a carrier film through a doctor blade method or the like and drying the same, separating a resulting ceramic green sheet from the carrier film, printing metal paste on the ceramic green sheet by screen printing or the like and drying the same, stacking a desired number of such ceramic green sheets to obtain a laminate, and pressurizing the laminate along the direction of stacking under appropriate conditions. This laminate is then cut if necessary, and thereafter fired to obtain a sintered ceramic laminate.
FIG. 11 is a sectional view showing a ceramic multilayer substrate 1 which is obtained basically through the aforementioned steps. Referring to FIG. 11, the substrate 1 comprises a plurality of ceramic layers 2 to 9 and internal electrodes 10 to 14 which are provided in interfaces between specific ones of the ceramic layers 2 to 9. A plurality of conductor films 16 to 18 are formed on one major surface 15 of the substrate 1. The conductor film 16 is electrically connected with the internal electrode 10 through a conductive internal through hole 19 passing through the ceramic layer 2. Further, two through hole connecting portions 20 and 21, for example, are defined to pass through the substrate 1, so that the through hole connecting portion 20 electrically connects the conductor film 17 and the internal electrodes 10, 12 and 14 with each other while the other through hole connecting portion 21 electrically connects the conductor film 18 and the internal electrodes 11 and 13 with each other.
When the ceramic multilayer substrate 1 shown in FIG. 11 is obtained by the aforementioned method, however, the following problems are caused:
A laminate of ceramic green sheets prepared to obtain the laminated structure of the ceramic layers 2 to 9 is pressurized in a stage before firing, as hereinabove described. Relatively high pressure is applied for such pressurization, and this can easily cause distortion of the green ceramic laminate, including the metal paste films which provide the internal electrodes 10 to 14. In general, such distortion is so non-uniform that it is difficult to obtain a green ceramic laminate, including the metal paste films, having the designed dimensions. Thus, such a green ceramic laminate frequently deviates from the designed dimensions, which reduces the yield of the manufacturing process. Non-uniform distortion of the green ceramic laminate in the aforementioned pressurizing step causes a significant problem particularly in the ceramic multilayer substrate 1 shown in FIG. 11, for example, which requires high position accuracy for the internal electrodes 10 to 14, the conductive internal through hole 19 and the like.
Further, the dried ceramic green sheets which provide the ceramic layers 2 to 9 are basically different in their material composition from the dried metal paste films which provide the internal electrodes 10 to 14. Sufficient junction strength cannot be attained by compressing such members made of basically different materials under mechanical pressure, and hence the finished product obtained upon firing has reduced rupture strength and reduce resistance against thermal shock. Even delamination may result in an extreme case.
For example in order to obtain a large capacitance in a laminated ceramic capacitor, a ceramic layer located between each pair of internal electrodes is most typically reduced in thickness. Referring to FIG. 12, a ceramic green sheet 22 is so thinned that its physical thickness 23 is substantially equal to the physical thickness 25 of a metal paste film 24 upon drying. When such ceramic green sheets 22 are stacked with each other, the thickness 25 of the metal paste film 24 partially formed on one major surface of each ceramic green sheet 22 cannot be neglected. As shown in FIG. 13, relatively large stresses remain in portions 27 and 28 corresponding to the edges of the metal paste films 24 upon pressurization of a laminate 26 of the ceramic green sheets 22. Such stress causes delamination or insufficient resistance against thermal shock after firing. This problem restricts the amount the thickness of the ceramic layers can be reduced for obtaining a large capacitance.
Another problem that arises in manufacturing a laminated ceramic capacitor, occurs when metal paste films for providing internal electrodes are printed on ceramic green sheets and dried, and then the ceramic green sheets are stacked with each other in an aligned state. However, when such ceramic green sheets are reduced in thickness as hereinabove described, the mechanical strength thereof is also reduced. Thus, it has been extremely difficult to register the ceramic green sheets as required for the steps of printing the metal paste films, stacking the ceramic green sheets and the like. Even if carrier films are employed for holding thin ceramic green sheets in such registration in order to compensate for the lack of mechanical strength, complicated and high-priced equipment is required for a registration mechanism, in order to handle basically thin substances.
FIGS. 14 and 15 show the so-called print lamination method, which is adapted to solve the aforementioned problem. This method basically repeats the steps shown in FIGS. 14 and 15. Referring to FIG. 14, for example, a squeegee 31 is driven in the direction of the arrow to act on a quantity of metal paste 30 which is placed on a screen 29, thereby to form a metal paste film 33 for providing an internal electrode. Then, as shown in FIG. 15, another squeegee 36 is driven in the direction of the arrow to act on a quantity of ceramic slurry 35 which is placed on a screen 34, thereby to form a green ceramic layer 37 for covering the metal paste film 33. Respective ones of such quantities of ceramic slurry 35 and metal paste 30 are repeatedly printed and dried to obtain a desired laminate.
However, the aforementioned print lamination method has the following problems:
First, a green ceramic layer formed by printing has a higher number of defects than a sheet formed through casting by the doctor blade method or the like. Thus, it is necessary to reduce the number of defects by repeating the printing step a plurality of times particularly for forming a green ceramic layer to be held between internal electrodes. This leads to reduction in productivity.
Further, the only means available for adjusting the thickness of a ceramic layer located between internal electrodes is merely controlling the printing conditions. Such control is relatively difficult in practice. In addition, the thickness of the ceramic layer cannot be easily managed if printing must be repeated a plurality of times in order to obtain one ceramic layer as hereinabove described. Thus, the resulting capacitance often deviates from a designed value, to cause reduction in yield.
Another problem is that in order to attain sufficient mechanical required for a laminated ceramic capacitor, upper and lower portions of a laminate must be covered with ceramic outer layers having no internal electrodes. However, the maximum thickness of a ceramic layer which can be formed by screen printing is several to tens of micrometers at the most. Thus, if hundreds of micrometers are required for each of the upper and lower outer layers, the number of printing times is extremely increased to cause reduction in productivity.
In both of the aforementioned methods of forming ceramic green sheets, namely through casting by the doctor blade method or the like, and by the print lamination method, pores and pinholes may be defined in the ceramic layers when their thickness is reduced. Thus, a capacitor may be reduced in voltage resistance, for example, or a short may be caused across the internal electrodes, in an extreme case.